Circuit arrangement for signaling in telecommunications networks

ABSTRACT

A circuit arrangement for signaling in telecommunications networks has a first connection and a second connection for a first and a second conductor, respectively, in a transmission line. The first connection is connected to the first input of a rectifier, and the second connection is connected to the second input of said rectifier. The positive output and the negative output of the rectifier are connected to one another via a variable resistance. A third connection for a third conductor in the transmission line can be connected via the series circuit formed by a switch and a first diode to the positive output of the rectifier, and via the series circuit formed by the switch and a second diode to the negative output of the rectifier.

BACKGROUND OF THE INVENTION

The invention relates to a circuit arrangement for signaling intelecommunications networks, as claimed in the preamble of patent claim1, and as is known from U.S. Pat. No. 4,190,745.

Private branch exchanges (PBX), digital multichannel transmissionsystems (DAML=Digital Added Main Line) or digital transmission systems(DLC=Digital Loop Carrier) have functions for signaling operatingstates. For this purpose resistances are connected between theindividual transmission lines by, for example, a private branchexchange. A switching center, which is connected to the private branchexchange, then applies a voltage between one line and the other lines,which are short-circuited, and measures the resultant currents throughthe lines. The switching center can use the measured currents to deducethe resistances connected between the individual transmission lines. Byvarying the resistance values, for example, the private branch exchangecan transmit information about operating states to the switching center.For example, in this case, it is possible to signal whether a voicetransmission or a data transmission is intended to take place.Alternatively, the switching center can find out whether thetransmission lines are faulty. In this case, however, differentresistance values must be provided and the resistance values must bematched to specific national requirements for the transmission lines,particularly when using public telecommunications network transmissionlines.

If there are three transmission lines, the measurement is carried out byusing switches, which are preferably in the form of relays, to connectresistances between the transmission lines. Two transmission lines arein each case short-circuited to one another, as a result of which tworesistances are in each case connected in parallel. The switching centerapplies a voltage to the remaining transmission lines. The resultantcurrents through the transmission lines and resistances are measured,and the resistance values are calculated from them. The calculationresults are also referred to as a signature. A signature makes itpossible, for example, to deduce the state of the transmission lines ora specific operating state. If one of the transmission lines is broken,then no current can flow via this line and the resistance valuedetermined by the switching center in this measurement, or thesignature, does not match the predetermined resistance value or thesignature for an uninterrupted line. A disadvantage in this case is thatthe relays for connecting the resistances between the transmission linesare expensive. A correspondingly large number of resistances must beprovided if there are a number of different signatures. Furthermore, aresistance must be provided for each transmission line, and theseresistances must be matched to the specific national requirements. It iseither necessary to construct different circuits for differentresistance values, or parallel resistance paths must be provided, with acorrespondingly large number of relays.

A circuit arrangement for detecting faults on a subscriber line is knownfrom U.S. Pat. No. 4,710,949, which uses switches that are sensitive tovoltage and current and are connected in the lines. A disadvantage inthis case is, however, that these switches are highly complex and arethus expensive. Furthermore, these switches can influence thetransmission of signals.

The invention is thus based on the technical problem of specifying acircuit arrangement for signaling in telecommunications networks which,firstly, can be matched to different requirements and, secondly, can beconstructed with little circuitry complexity.

This object is achieved by a circuit arrangement for signaling intelecommunications networks having the features of patent claim 1.Advantageous refinements of the circuit arrangement can be found in therespective dependent claims.

SUMMARY OF THE INVENTION

A circuit arrangement for signaling in telecommunications networks has afirst connection and a second connection for a first and a secondconductor, respectively, in a transmission line. The first connection isconnected to the first input of a rectifier, and the second connectionis connected to the second input of said rectifier. The positive outputand the negative output of the rectifier are connected to one anothervia a variable resistance. A third connection for a third conductor inthe transmission line can be connected via the series circuit formed bya switch and a first diode to the positive output of the rectifier, andvia the series circuit formed by the switch and a second diode to thenegative output of the rectifier.

The control loop, the rectifier and the transistor and resistance areused for setting a line impedance during normal operation of a linecircuit which, for example, links a digital subscriber circuit to ananalog transmission line. Thus, advantageously, the invention provides acircuit arrangement for signaling in telecommunications networks withlittle additional circuitry complexity—two diodes and one switch.Instead of having to provide a specific resistance for testing eachconductor in a transmission line, which is connected by means of a relaybetween two conductors in the transmission line for testing, alreadyexisting circuits are provided with additional circuits for testing thetransmission line.

In one preferred embodiment, a control loop is provided to which a firstvoltage, which is produced at the positive output of the rectifier, anda second voltage, which is produced at the negative output of therectifier, are supplied. The control loop produces a control voltagewhich controls the variable resistance. The transfer function of thecontrol loop is variable. A particularly advantageous feature in thiscase is that the capability to vary the transfer function of the controlloop makes it possible to vary the variable resistance, and thus theresistance between two conductors in the transmission line, duringtesting. The circuit arrangement can thus be matched to specificnational requirements, and different signatures can be produced, withoutchanging the circuitry.

It is particularly preferable for the variable resistance to have atransistor.

The cathode of the first diode is preferably connected to the positiveoutput of the rectifier, and the anode of the second diode is preferablyconnected to the negative output of the rectifier.

The control loop preferably has a digital filter, with the transferfunction of the control loop being variable by programming the filtercoefficients of the digital filter. It is particularly preferable inthis case for the digital filter to be formed by an appropriatelyprogrammed digital signal processor.

In one preferred embodiment, the control loop has an analog integratorcircuit which is connected upstream of the transistor and integrates thedifference between a first input voltage and a second input voltage, andwhose output signal controls the transistor.

It is particularly preferable for the transistor to be an n-channelMOSFET.

The circuit arrangement for signaling in telecommunications networks isalso suitable for being added to an integrated circuit, by virtue of theuse of electronic components. In this case, of course, all the othercircuits, such as the control loop, the rectifier and the transistor andresistors, can also be integrated at the same time.

Further advantages, features and application options for the inventionwill become evident from the following description of exemplaryembodiments in conjunction with the drawing. In the drawing:

BRIEF SUMMARY OF DRAWINGS

FIG. 1 shows an exemplary embodiment of a circuit arrangement forsignaling in telecommunications networks, and

FIG. 2 shows three different operating states of the circuitarrangement, in order to detect line interruptions.

DETAILED DESCRIPTION

The circuit arrangement illustrated in FIG. 1 for signaling intelecommunications networks has three connections a, b and c, which canbe connected to a three-conductor transmission line. The transmissionline may, for example, comprise a two-conductor a/b conductor fortransmitting signals, and a ground line c.

The circuit arrangement for signaling is used in a chip set forconnection of a digital subscriber circuit (DSL=Digital Subscriber Line)to an analog telephone line (transmission line). The chip set in thiscase converts the signals on the analog telephone line to digitalsignals for further processing by the digital subscriber circuit and,conversely, converts digital signals to analog signals for the digitalsubscriber circuit for transmission via the analog telephone line.

A switching center can apply various signaling voltages via thetransmission line. To do this, the switching center short-circuits twoof the three conductors in the transmission line. Furthermore, thecircuit arrangement for signaling connects a resistance between theremaining conductor and the two short-circuited conductors. Theswitching center applies a voltage between the remaining conductor andthe two short-circuited conductors. A current flowing via the conductorsis measured by the switching center, and this is used to calculate aresistance. If there are no faults on the lines, the calculatedresistance will be within a specific predetermined range. In this case,the predetermined range is defined by specific national requirements.

The connections a and b of the circuit arrangement for signaling areconnected to a first input and a second input, respectively, of a bridgerectifier 1. A first output 12 and a second output 13 of the bridgerectifier can be connected, respectively, via a first diode D1 and aswitch S to the third connection c of the circuit arrangement forsignaling and via a second diode D2 and the switch S to the thirdconnection c of said circuit arrangement for signaling. The n-connectionof the first diode D1 is in this case connected to the first output 12of the rectifier 1. The p-connection of the second diode D2 is connectedto the second output 13 of the rectifier 1.

The first output 12 of the bridge rectifier 1 is connected to areference ground potential VSS via the load path of a transistor T1. Thesecond output 13 of the bridge rectifier 1 is connected to the referenceground potential VSS via a resistance R1.

A voltage which is present between the connections a and b is rectifiedby the bridge rectifier 1. A rectified positive voltage Va is producedat the first output 12 of the bridge rectifier 1. A rectified negativevoltage Vb is produced at the second output 13 of the bridge rectifier1.

The rectified positive voltage Va, whose voltage levels are high, issubdivided via a voltage divider R2 and R3 to produce a lower voltage,in order to avoid overdriving the downstream circuits, in which thesignal voltage levels are only low in comparison to the rectifiedpositive voltage.

After being divided, the positive voltage Va and the negative voltage Vbare supplied to a subtraction circuit 5, at whose output a differencevoltage Vab is produced.

The difference voltage Vab is then sampled by an analog/digitalconverter 2 at a sampling rate fs and is converted to a digital signalV′ ab.

The digital output signal V′ ab from the analog/digital converter 2 issupplied to a digital filter 3. The digital filter 3 can be programmedby a digital control device 10 for matching to specific nationalrequirements and, for this purpose, has a programmable transfer functionk. In this case, the digital filter 4 may be in the form of digitalhardware, in which the coefficients are programmable. The digital filtermay likewise be in the form of a signal processing algorithm in adigital signal processor, in which case the filter function can beadjusted by means of variables.

The output signal VSI from the digital filter 4 is converted by adigital/analog converter 4 to an analog signal VI.

The analog signal VI is supplied to a first input of an analogintegrator circuit 6. The negative voltage Vb is supplied via a secondinput to the analog integrator circuit 6. The analog integrator circuit6 uses the two input signals to form a difference, which is thenintegrated. The voltage VSt is produced at the output of the analogintegrator circuit 6, and is passed to the control connection of thetransistor T1. A line current I is set via the transistor T1. The analogintegrator circuit 6 integrates the difference voltage VI−Vb until thedifference voltage VI−Vb becomes zero. Thus, from Vb=R1*I=VI, it ispossible to derive a conductance value GM=I/VI=1/R1 for the analogintegrator circuit 6.

FIG. 2 shows three different cases for testing the transmission line.Two conductors in the transmission line are in each case short-circuitedfor each case, corresponding to the two illustrated connections, whichare annotated by means of a dashed line as being connected to oneanother. The switching center applies a voltage between theshort-circuited connections and the remaining conductor, or theremaining connection.

Case 1: The connections a and c are short-circuited. A first voltage V1is applied between the connection b and the short-circuited connectionsa and c. The switch S is opened. A first current I1 flows via the pathmarked by a bold line. A resistance V1/I1 can be calculated from thefirst voltage V1 and the first current I1, and this is governed by theresistance of the load path through the first transistor T1 and theresistance R1.

Case 2: The connections b and c are shorted-circuited. A second voltageV2 is applied between the connection a and the short-circuitedconnections b and c. The switch S is opened. A second current I2 flowsvia the path marked by a bold line. A resistance V2/I2 can once again becalculated from the second voltage V2 and the second current I2, andthis is governed by the resistance of the load path through the firsttransistor T1 and the resistance R1.

Case 3: The connections a and b are short-circuited. A third voltage V3is applied between the connection c and the short-circuited connectionsa and b. In this case, the switch S is closed. A third current I3 flowsvia the path marked by a bold line. A resistance V3/I3 can once again becalculated from the third voltage V3 and the third current I3, and thisis governed by the resistance of the load path through the firsttransistor T1 and the resistance R1.

One conductor in the transmission line is tested in each of the threecases. If the calculated resistance differs from a predetermined valuein one of the three cases, then there is a faulty transmission line.Since the calculated resistance value depends on the resistance of theload path through the first transistor T1 and the resistance R1, theresistance of the load path through the transistor T1 can be matched to(for example) specific national requirements by programming the transferfunction of the digital filter 3. For this purpose, specific nationalvalues for the predetermined resistance can be stored in a memory 11. Tothis end, the digital control device 10 reads from the memory 11 thevalues required for programming the specific national resistance, andchanges the programming of the digital filter 3 accordingly.

What is claimed is:
 1. A circuit arrangement for signaling intelecommunications networks, with a first connection (a) and a secondconnection (b) for a first and a second conductor, respectively, in atransmission line being connected to the first input and to the secondinput, respectively, of a rectifier, and with the positive output andthe negative output of the rectifier being connected to one another viaa variable resistance wherein a third connection (c) for a thirdconductor in the transmission line can be connected via the seriescircuit formed by a switch and a first diode to the positive output ofthe rectifier, and via the series circuit formed by the switch and asecond diode to the negative output of the rectifier.
 2. The circuitarrangement as claimed in claim 1, wherein a control loop is provided towhich a first voltage, which is produced at the positive output of therectifier, and a second voltage, which is produced at the negativeoutput of the rectifier, are supplied, and the control loop produces acontrol voltage which controls the variable resistance, and with thetransfer function of the control loop being variable.
 3. The circuitarrangement as claimed in claim 1, wherein the variable resistance has atransistor.
 4. The circuit arrangement as claimed in claim 3, whereinthe cathode of the first diode is connected to the positive output ofthe rectifier, and the anode of the second diode is connected to thenegative output of the rectifier.
 5. The circuit arrangement as claimedin claim 2, wherein the control loop has a digital filter, with thetransfer function of the control loop being variable by programming thefilter coefficients of the digital filter.
 6. The circuit arrangement asclaimed in claim 5, wherein the digital filter is formed by anappropriately programmed digital signal processor.
 7. The circuitarrangement as claimed in claim 3, wherein the control loop has ananalog integrator circuit which is connected upstream of the transistorand integrates the difference between a first and a second inputvoltage, and whose output signal controls the transistor.
 8. The circuitarrangement as claimed in claim 3, wherein the transistor is ann-channel MOSFET.